Tornado and Thunder Power Cables

It’s just that home theater receivers aren’t exactly known for being low noise / distortion enough for some to be considered ‘hi-fi’ at least for clean 2ch music listening (but Marantz is def good consumer-level stuff)… So if this is about lowering noise or cleaning up distortions, you may get better results going with lower distortion ICs first. This will also help aftermarket power cords have a bigger perceived difference. 
 But even better money spent is the recommendation to get a good hifi 2ch preamp, with HT bypass function, for 2ch music listening. Then you’ll be stepping into the lower noise / cleaner signal realm and ICs and PCs will make a greater difference. 
 Just as an example: Not affiliated but I recently tried some Hapa Audio copper cables and was amazed at how clean of a sound and less distorted they were over my other similarly priced IC options.

The AQ thunders are great. I have one on my regenerator and it makes a huge difference. The ones above who think that power cables don't make a difference when you have a "good" power supply, is severely mistaken and hasn't bothered to attempt it on his equipment. Cables definitely impact sound. 

I will agree with the others to where your cables are outclassing your amp. You might get more of a benefit putting that +$4k into an amp. 

Rotel amps are definitely not one that is recommended for 2 channel listening

Preamp- Marantz AV8805: $4500
Power amp(2 channel)- Rotel RB1590: $3700
Power amp(5 channel)- Rotel RMB1585: $3700

I’m a little bit confused ? What am I missing here? The OP got $11,900 worth of gear and his gear is NOT recommended for 2 channel listening!? We all should be so lucky!

I say go for exactly what you want - the Audio Quest Thunder at around $900 each is perfectly reasonably priced - for YOUR budget. And do NOT believe the naysayers that say there will be no improvement in sound quality. You will get quieter and blacker backgrounds and better and less fatiguing sound coming from your speakers.

Also, I think it maybe a good idea using all Audio Quest PC’s as they will have the same sound signature and blend in well with each other, especially if you are planning to get a Audio Quest conditioner at a later date.

And one more thing, don’t stop with other upgrades and tweaks. Damping, IC's, speaker wire and fuses to name a few.

Sheesh! What a tough crowd!

As an electrical engineer I always wonder about the choices people make in searching for better sound. 

I will state that the power supply is where the magic begins. 

A robust well designed PS is key. 

Most quality equipment has that. 

What happens before the power supply is moot. 

If this was the case,  why not replace the wires from the outlet receptical to the fuss box?

Then back from there? 

There is no downside from upgrading a power cable,  more secure connection and getting the perfect length but don't expect it to affect the sound quality. 

Invest your money into good speakers first.  Then a good source Then compatible equipment in between. 

Room acoustics and maybe room correction software. 

Just my 2cents.

As an electrical engineer I always wonder about the choices people make in searching for better sound.

Have you tried using a better PC than the supplied stock cable?
Room correction software does work but many software packages not only color but degrade the sound quality also. Just my 2 cents.